The present invention relates to a refrigerant circulating apparatus having a refrigerant circuit in which a refrigerating machine oil is difficult to dissolve in a refrigerant as in a case where, for example, a hydrofluorocarbon- (HFC-) based refrigerant is used as a refrigerant and an alkylbenzene-based oil as a refrigerating machine oil.
An example of a conventional refrigeration and air-conditioning cycle apparatus is shown in FIG. 20. In a case where a refrigerating machine oil such as alkylbenzene, which has weak compatibility with respect to a hydrofluorocarbon- (HFC-) based refrigerant, is used as shown in Japanese Patent Application Laid-Open No. 208819/1995, the return of oil from an accumulator provided on the low-pressure side where the solubility of the refrigerating machine oil in the liquid refrigerant declines has hitherto been an important problem in the reliability of a compressor. FIG. 20 shows a refrigeration and air-conditioning cycle apparatus in which an HFC-based refrigerant and an oil having weak solubility are used as a refrigerant and a refrigerating machine oil, respectively, wherein reference numeral 1 denotes a compressor for compressing a refrigerant gas; 2, a four-way valve having the function of reversing the flowing direction of the refrigerant; 5, a pressure reducing device; 7, an accumulator for accumulating surplus refrigerant; 14, a refrigerating machine oil stored in the compressor 1 to effect the lubrication of sliding portions of the compressor 1 and the sealing of a compression chamber; 52, a condenser for condensing a high-pressure refrigerant gas discharged from the compressor 1; and 55, an evaporator.
The refrigerating machine oil with weak solubility used in this refrigeration and air-conditioning cycle apparatus, e.g., alkylbenzene, has nonsolubility or very weak solubility with respect to an HFC-based refrigerant, with its rate of solubility in the liquid refrigerant under the conditions of condensing pressure and condensing temperature being 0.5-7 wt %, its rate of solubility in the liquid refrigerant under the conditions of evaporating pressure and evaporating temperature being 0-2.0 wt %, and its specific weight in the temperature range of -20.degree. C. to +60.degree. C. being a value smaller than the specific weight of the liquid refrigerant at the same temperature and under saturated vapor pressure.
Next, a description will be given of the behavior of the refrigerating machine oil. The high-pressure refrigerant gas compressed by the compressor 1 is discharged to the condenser 52. Most of the refrigerating machine oil 14 used for lubricating the compressor and for sealing the compression chamber returns to the bottom of a hermetic container, but the refrigerating machine oil having an oil circulation rate of 0.3 to 2.0 wt % or thereabouts is discharged together with the refrigerant from the compressor 1. The pipe diameter of the condenser 52 where the refrigerant gas flows is set so as to secure a flow rate of the refrigerant gas sufficient to convey the refrigerating machine oil downstream. Although most of the refrigerant liquefies in the vicinity of an outlet of the condenser 52 and the in-pipe flow rate declines appreciably, since the refrigerating machine oil has weak solubility with respect to the condensed liquid refrigerant, the refrigerating machine oil dissolves in the liquid refrigerant and is conveyed to the pressure reducing device 5. The temperature and pressure of the refrigerant decline appreciably in a region downstream of the pressure reducing device 5, and the solubility characteristic of the refrigerating machine oil changes to nonsolubility or very weak solubility with respect to the liquid refrigerant. However, the refrigerating machine oil is conveyed to the accumulator 7 since the flow rate of the refrigerant increases abruptly due to the gasification of part of the liquid refrigerant which occurs in the region downstream of the pressure reducing device 5, and since the pipe diameter of the evaporator 55 in the next stage is set so as to secure a flow rate of the refrigerant gas sufficient to convey the refrigerating machine oil downstream. Since the solubility of the refrigerating machine oil in the liquid refrigerant under the conditions of evaporating pressure and evaporating temperature is nil or very weak, the refrigerating machine oil 81 forms a separate layer over the liquid refrigerant 13 inside the accumulator 7. For this reason, the structure provided is such that a plurality of oil returning holes 72a, 72b, 72c, and 72d having different heights from a lower end 7a of the accumulator are provided in a lead-out pipe 71 for leading the refrigerant from inside to outside the accumulator, thereby promoting the return of the oil to the compressor 1.
As another example of the conventional refrigeration and air-conditioning cycle apparatus, a refrigeration and air-conditioning cycle apparatus disclosed in Japanese Patent Application Laid-Open No. 19253/1989 is shown in FIG. 21. Reference numeral 1 denotes the compressor for compressing a refrigerant gas; 52, the condenser for condensing the high-pressure refrigerant gas discharged from the compressor 1; 31, a pre-stage pressure reducing device; 54, a receiver for accumulating surplus refrigerant; 32, a post-stage pressure reducing device; 55, the evaporator; and 2, the four-way valve having the function of reversing the flowing direction of the refrigerant.
Next, a description will be given of the operation of this refrigeration and air-conditioning cycle apparatus. The high-pressure refrigerant gas compressed by the compressor 1 passes through the condenser 52 while becoming liquefied, is then subjected to pressure reduction by the pre-stage pressure reducing device 31, and enters the receiver 54. Here, by controlling the pressure reducing devices disposed respectively before and after the receiver 54, the surplus refrigerant is accumulated in correspondence with the condition of the load of the apparatus, thereby optimizing the performance and efficiency and ensuring the reliability of the compressor. The liquid refrigerant which flowed out from the receiver 54 is further subjected to pressure reduction to the level of necessary evaporating pressure, then passes through the evaporator 55, and is sucked into the compressor 1.
In the refrigeration and air-conditioning cycle apparatus shown in FIG. 20 and cited as a conventional example which uses a hydrofluorocarbon- (HFC-) based refrigerant as a refrigerant and an alkylbenzene-based oil as a refrigerating machine oil, the following problem is encountered in the case where a large amount of surplus refrigerant is accumulated in the accumulator 7 and the liquid level has become high.
First, although the refrigerating machine oil 81 which cannot be dissolved in the liquid refrigerant is separated from the liquid refrigerant 13 and is accumulated in an upper layer of the two separated layers, since the force of suction from the upper holes 72c and 72d declines as compared with that from the hole 72a provided in a lower end of the lead-out pipe 71 among the oil holes 72 provided in the lead-out pipe 71 inside the accumulator 7, only the liquid refrigerant 13 in the lower layer flows into the lead-out pipe 71, and the refrigerating machine oil 81 in the upper layer scarcely flows into the lead-out pipe 71. Therefore, the refrigerating machine oil 81 is accumulated in a large amount inside the accumulator 7, with the result that the refrigerating machine oil 81 in the compressor 1 is depleted, possibly causing faulty lubrication. Next, when the liquid level of the liquid refrigerant becomes high, since the liquid refrigerant is sucked from the plurality of oil returning holes in the lead-out pipe 71, a large amount of liquid refrigerant returns to the compressor 1, which possibly results in a sudden pressure rise in the compression chamber due to the supply of the noncompressive liquid refrigerant to the interior of the compression chamber. In addition, since the liquid refrigerant discharged from the compression chamber is detained in the hermetic container of the compressor, the liquid refrigerant instead of the refrigerating machine oil 81 is supplied to lubricating element portions, which can cause seizure and the like of the bearing of the compressor 1 and sliding portions of compressing elements, thereby leading to a decline in the reliability. In addition, if the diameters of the oil returning holes 72 are set to be small so as to prevent a large amount of liquid refrigerant from returning to the compressor 1, the return of the refrigerating machine oil 81 is further aggravated, and dust, impurities, and the like in the circuit are liable to clog the oil returning holes 72.
With the refrigeration and air-conditioning cycle apparatus shown in FIG. 21 and cited as a conventional example, the apparatus can be operated without a problem in a case where a refrigerating machine oil having compatibility with a refrigerant is used, but if a refrigerating machine oil having noncompatibility or weak compatibility is used, the refrigerating machine oil which is nonsoluble in the liquid refrigerant is separated in an upper layer and is detained inside the receiver 54 under the operating conditions in which the rate of oil circulation is large, and the refrigerating machine oil inside the compressor 1 is depleted, thereby possibly causing faulty lubrication.
Conventionally, when an airtight test is performed in the process of manufacturing the compressor using R.22 as a refrigerant, a discharge pipe and a suction pipe are closed by jigs, and the airtight test is performed under the pressure of 28 kgf/cm.sup.2 G. However, in a case where a high-pressure refrigerant such as R.410A is used as the hydrofluorocarbon- (HFC-) based refrigerant, the pressure corresponding to the refrigerant in the case of R.410A is considerably high at 45 kgf/cm.sup.2 G, with the result that there has been a possibility of the jigs from coming off when the airtight test is performed.